Humans Have Ancient Armored Fish to Thank for Their Sex Lives

If you've ever complained that your bedmate's behavior was similar to that of a dead fish, it might not be their fault. As it turns out, our most basic sexual behavior may have originated more than 385 million years ago with ancient armored fishes called antiarchs.

New research published in the journal Nature on Sunday posits that antiarchs — which were some of the first vertebrates with jaws — were also the first creatures to use internal fertilization to reproduce. After examining fossil data, scientists found that males had L-shaped copulatory claspers (which were almost as long as their bodies) and females had structures that resembled genital plates.

The thinking goes like this: the claspers were inserted into the female's genital plates, which were small enough to help lock the clasper into place. Sperm was then transferred between then two, allowing fertilization — and reproduction — to occur.

Alas, there wasn't a whole lot of creativity involved for these little guys — they relied on one position and one position only.

"When you look at the shape of these structures they can't possibly do anything in a missionary position," palaeontologist John Long, a lead researcher on the study, told the Sydney Morning Herald. "We've printed out 3D models of these fish and I can play mating games with them and the only way possible they can do it is sideways, square dance style, with their little arms entangled," he said.

Imperial College London paleontologist Martin Brazeau wondered how scientists hadn't stumbled upon this before. "Microbrachius has been known to science for nearly 130 years. How did we overlook this?" Brazeautold National Geographic. "We've been assuming the evidence simply isn't there, but it's been right under our noses for a long time."

But don't fish reproduce externally? Today's fish as we know them participate in external fertilization, which leads one to wonder what happened between 385 million years and today.

Unfortunately, Long and company don't have the answer. "We don't know," said Long. "It could be a change in environment or a special adaptation that happened at the time."

As Vox notes, external reproduction seems to have worked out pretty well for our fish friends — they've survived this long, after all. Long suggested that the gene for the mating body parts likely got switched on and off as the fish evolved. "Once that gene was set in the vertebrate body plan, it could come back later," he said.

As funny as it is, however, the discovery is truly important. "We've defined the point in evolution when the origin of internal fertilization in all animals began and that's a really big step," said Long. "In terms of evolution, this is the very earliest act of copulation that we know of."

Then again, the act also sounded quite stressful — the Guardian dryly notes that it "was not the smoothest of affairs," which was likely why the males had little arms to help them get the job done.

So, next time you're getting frisky, just think about where the act originated — with male sex organs almost as long as the body itself and female sex organs that were compared to "tiny cheese graters" — and thank evolution for making it a little easy for all of us.